Digital burst frequency translator

ABSTRACT

A device for detecting and measuring analog coherent frequency bursts includes circuitry for digitally translating the bursts upon their detection. Based on a frequency range estimate, a sampling frequency is selected to govern conversion of the analog burst to a sequence of digital values, which are stored to a sequential memory. The digital values later are read out of the memory to a digital-to-analog converter at a predetermined read frequency. The D/A converter output is low-pass filtered to provide a reconstructed analog burst. Sampling rates are selected to limit the frequency range of reconstructed bursts to a single octave, which enables subsequent signal processing with simpler circuitry, for reduced cost and improved accuracy and reliability of measurements obtained.

BACKGROUND OF THE INVENTION

The present invention relates to circuitry for processing electricalsignals, and more particularly to circuitry for changing the frequencyof such signals to enhance their subsequent further processing andanalysis.

Over the years, numerous instruments have been developed for measuringphysical phenomena by detecting energy generated due to the phenomenaand converting the energy to electrical signals. The electrical signalsare processed to obtain information on a wide variety of physicalcharacteristics, e.g. the location, size and velocity of an object, orits index of refraction, transmissivity or surface characteristics. Inmany applications, signals occur intermittently and randomly and vary intheir amplitude and duration. Further, the signals may be expected tooccur over a wide frequency bandwidth in which frequencies near theupper end of the bandwidth are at least several orders of magnitudegreater than frequencies at the lower end.

While such applications can involve radar and sonar, the presentinvention is more directly concerned with laser Doppler velocimetry(LDV), also known as laser Doppler anemometry. LDV systems are known fortheir utility in measuring instantaneous velocities of discrete elementsin two-phase flows, e.g. liquid sprays in air, or fine particles influid streams. In a conventional LDV apparatus, one or more pairs ofspatially separated laser beams intersect one another and interfere withone another to form a measuring volume. The measuring volume istypically quite small, and the concentration of particles sufficientlylow, so that at any given time no more than one particle is within themeasuring volume. One or more photodetectors receive the coherent lightscattered by particles passing through the measuring volume. The Dopplerfrequency, obtained by measuring the electrical signal generated as afunction of light received by the photodetector, measures particlevelocity and thus the velocity of the particle-carrying medium. Dopplerfrequency is proportional to particle velocity.

Laser phase Doppler systems are closely related to LDV systems andemploy a phase difference between two separate Doppler frequency signalsto determine the size of a moving particle. More particularly, two ormore photodetectors receive light scattered by a particle passingthrough a measuring volume. The photodetectors receive light fromdifferent locations relative to the measuring volume. The difference inphase between signals from the photodetectors provides the particle sizeinformation.

Particles, especially spherical particles, tend to scatter light in alldirections and thus lend themselves well to analysis by laser Dopplertechniques. However, physical characteristics of systems under analysiscan lead to problems in analyzing the resultant electrical signals.Given the typically low particle density and small size of the measuringvolume, particles traverse the measuring volume individually and inintermittent, random fashion. The resultant electrical signal is acomposite of background noise and occasional coherent frequencycomponents, known as "bursts" superimposed on the background noise.Thus, signal processing circuitry should be capable of distinguishingthe coherent frequency bursts from noise to avoid wasteful attempts toanalyze the noise. The coherent frequency bursts tend to be non-uniformin amplitude, frequency and duration. Signal amplitudes vary withparticle size, but also with varying tendencies of particles to absorbrather than scatter the laser energy. Signal frequencies can vary overorders of magnitude, particularly in turbulent flow systems. The lengthor duration of the bursts can vary considerably, even under conditionsof uniform particle size and velocity, depending upon whether a particleis substantially centered as it traverses the measuring volume.

Known processing techniques, e.g. involving LDV counters, are capable ofprocessing coherent frequency bursts in real time. However, the signalis prone to distortion and noise that depends on signal frequency,amplitude and processing bandwidth. Thus, the nature and degree ofdistortion is difficult to predict.

Alternatively, the electrical signals can be sampled and converted tocorresponding digital signals and then processed digitally. Whiledigital sampling and processing do not eliminate distortion, thedistortion is more predictable. Typical digital processing techniquesinclude signal correlation and fast Fourier transform. A disadvantage ofthis approach is that sampling and processing time can considerablyexceed burst durations, seriously decreasing the rate at which the burstsignals can be processed.

Given the wide range of possible burst signal frequencies, circuitry forprocessing the signals must be capable of functioning over a broadfrequency band. This requirement calls for circuitry which is morecomplex, more costly and less reliable than circuitry tailored to handlea narrower frequency bandwidth.

To address this difficulty, analog and digital processors have been usedto heterodyne, or translate, the signal frequency. Typically, analogmixing techniques are employed to limit the frequency range of signalssubject to further processing, thus to reduce the complexity ofprocessing circuitry. Analog mixing, however, is subject to reducedsignal-to-noise ratio due to mixer insertion loss and post-mixeramplification and filtering. Intermodulation distortion and harmonicdistortion also increase noise.

Therefore, it is an object of the present invention to provide circuitryfor real time processing of analog signals, while minimizing noise andsignal distortion.

Another object of the invention is to provide a process for rapidlyadjusting the frequency bandwidth of electrical signals before furtherprocessing of the signals.

A further object is to provide a means for receiving signals over abroad range of frequencies and conditioning the signals for furtherprocessing within a considerably narrowed frequency bandwidth.

Yet another object is to provide simpler, less costly and more reliablecircuitry for frequency translation of coherent frequency bursts.

SUMMARY OF THE INVENTION

To achieve these objects, there is provided a process for measuring acoherent frequency analog signal, including the following steps:

a. receiving an analog coherent frequency signal, and generating afrequency range value indicating an approximate frequency of thereceived analog coherent frequency signal;

b. predetermining a plurality of sampling rates, each sampling ratecorresponding to a different one of a plurality of frequency bandwidthsegments that together comprise a frequency bandwidth for receiving theanalog coherent frequency signal;

c. selecting one of the plurality of sampling rates, based on thefrequency range value;

d. converting the analog coherent frequency signal to a first digitalsignal by sampling the analog coherent frequency signal at the selectedsampling rate;

e. storing the first digital signal to a memory as a sequence of digitalvalues, each digital value corresponding to one sample of the analogcoherent frequency signal;

f. reading the first digital signal out of the memory in the sequenceand at a predetermined read frequency to generate a second digitalsignal within a reduced frequency bandwidth; and

g. converting the second digital signal to an analog signal, thus togenerate a reconstructed analog low noise signal and filtering with abandpass filter corresponding to the analog coherent frequency signal.

Preferably, each bandwidth segment is predetermined as to range, suchthat the ratio of its highest frequency to its lowest frequency is atmost two. Accordingly each bandwidth segment has a range of one octaveor less. The full bandwidth typically is a decade, i.e. with the highestfrequency being about ten times the lowest frequency of the range. Acontroller can be provided to select one of several sampling rates, eachrate associated with one of the bandwidth segments. Consequently,despite a broad range of initial signal frequencies, appropriatematching of sampling rates and estimated frequencies controls andconfines the relationship of the signal and sampling frequencies. Forexample, the number of samples per cycle of the analog coherentfrequency signal can be confined to within a predetermined range such as5-10 samples/cycle.

Conversion to the first digital signal proceeds at the selected samplingrate, and preferably comprises generating multiple eight-bit binarywords, each binary word corresponding to one of the samples andrepresenting a digital value. The first digital signal is stored as asequence of the binary words.

The second digital signal is generated by reading the digital values outof the memory, in the sequence, and at the predetermined read frequency.While the read frequency most directly controls the number of digitalvalues or samples read per second, it also controls the cycles persecond, based on the aforementioned relationship of samples and cyclesof the original signal. More particularly, if appropriate selection ofsampling rates confines the number of samples per cycle to a range of5-10, the resulting digital signals read out of the memory fall within aone octave frequency range. The reconstructed analog signals, likewise,lie within this limited frequency bandwidth. The predetermined readfrequency further positions the narrowed frequency bandwidth, i.e. atleast approximately determines the maximum and minimum frequencieswithin the bandwidth.

To further enhance signal processing accuracy, the reconstructed analogsignal can be low pass filtered (preferably approximately at the Nyquistvalue) before it is processed. The available range at which coherentfrequency bursts are processed can be enhanced considerably by band-passfiltering of the incoming analog coherent frequency signal. In aspecific application, sixteen band-pass filters are used in combinationto sense frequencies varying from 300 Hz through 100 MHz.

The manner in which the reconstructed analog signal is processed dependsupon the requirements of the physical system. In a velocity measurementsystem, the reconstructed analog signal is processed to accuratelydetermine its frequency. This measuring frequency is combined with theknown selected sampling rate to accurately determine the frequency ofthe original analog coherent frequency signal, i.e. the burst frequency.Then, particle velocity is determined as a function of the burstfrequency.

Alternatively, particle sizes can be determined based on differences inphase between several frequency bursts based on the same particle. Thisrequires two or more photodetectors angularly spaced apart from oneanother, each generating an analog coherent frequency burst responsiveto receiving energy scattered by the particle. The burst signals areseparately converted and reconstructed as described above, forprocessing within a narrowed frequency bandwidth. A phase-detectingmeans is employed for determining a phase difference between thereconstructed signals. The particle size is determined as a function ofthe phase difference.

Thus in accordance with the present invention, analog coherent frequencysignals can be sensed over a broad frequency bandwidth, converted todigital information, then reconverted to provide reconstructed analogsignals confined to a narrow frequency bandwidth for processing. When soconfined as to their frequencies, the reconstructed signals lendthemselves to processing by analog techniques employing simpler, lesscostly circuit components, high processing speed, and enhancedreliability due to reduced distortion and noise. This considerablyimproves the accuracy of measurements obtained in laser phase Dopplerand laser Doppler velocimetry systems, and similarly can improve radarand sonar applications involving wide frequency ranges.

IN THE DRAWINGS

For a further appreciation of the above and other features andadvantages, reference is made to the following detailed description andto the drawings, in which:

FIG. 1 is a schematic view of a laser Doppler measurement systemincluding signal processing circuitry;

FIG. 2 is a more detailed view of the optical elements of the laserDoppler system;

FIG. 3 is an enlarged partial view of FIG. 2;

FIG. 4 is a schematic view of a bandpass filtering device shown in FIG.1;

FIG. 5 is a more detailed schematic view of a frequency translator shownin FIG. 1;

FIGS. 6 and 7 are timing diagrams, showing an electrical signal atvarious stages of its processing by the frequency translator;

FIG. 8 is a schematic view of an alternative embodiment frequencytranslator employing selectively phase-shifted A/D converters andmemories;

FIG. 9 schematically shows part of a size-measuring device constructedin accordance with the present invention; and

FIG. 10 schematically illustrates a part of velocity measurement deviceconstructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, there is shown in FIG. 1 a laser Dopplersystem 16 for generating electrical signals based on particles or otherlight-scattering elements moving through a particle measurement volume.The system processes the signals to determine sizes, velocities, orother information about the moving particles. The system includesoptical apparatus 18 for generating an electrical signal as a functionof sensed particles. Typically, particles are conveyed by a medium,usually a gas or liquid, through a measuring volume defined byintersecting coherent energy beams. The particle concentration issufficiently low, and the measuring region sufficiently small, tovirtually eliminate the possibility of more than one particle beingwithin the measuring region at any given time. Accordingly, the signalgenerated by apparatus 18 is a composite signal that includes analogcoherent frequency components known as "bursts", and background noise.Particles cause bursts of widely varying amplitudes, frequencies anddurations, depending on their size, speed, index of refraction,absorption characteristics and the extent to which they are "centered"as they traverse the measuring volume.

The composite signal, including the bursts and background noise, isprovided to an amplifier 20 and the amplified signal then provided to abandpass filtering device 22, where the signal is bandpass filtered tolimit its frequency range. The filtered signal is provided to a burstdetector 24 and a frequency translator 26.

Burst detector 24 can be of the type described in U.S. Pat. No.4,973,969 (Jensen), issued Nov. 27, 1990 and incorporated by referenceherein. Burst detector 24 provides a signal burst gate 27 to frequencytranslator 26 indicating that a burst has been detected, and a frequencyrange signal 29 indicating an approximate frequency of the burst.

Based on the estimated frequency, frequency translator 26 selects asampling rate for sampling the analog output of filtering device 22 andconverting that output to a digital signal. Within the frequencytranslator, the digital signal is selectively modified as to itsfrequency and converted to a reconstructed analog signal correspondingto the filtering device output.

The reconstructed analog signal, confined within a frequency bandwidthof about one octave, is provided to signal processing circuitry 28 fordetermining the signal frequency, phase or other characteristiccorresponding to the physical phenomenon being measured. Processingcircuitry 28 provides an output to a microprocessor 30, which can be apersonal computer. The corresponding physical measurement values can beshown on a display terminal 32 connected to the microprocessor.

FIG. 2 shows the optical apparatus in greater detail. To measure onevelocity component and/or phase, a laser 34 and appropriate collimatinglens 38 generates the coherent energy in a beam 36. Beyond lens 38, abeam splitter 40 receives the collimated beam and generates a pair ofcollimated laser beams 36a and 36b, directed longitudinally parallel toa beam axis 41, toward a focusing lens 42. Beams 36a and 36b aretransversely spaced apart and define a beam plane.

Focusing lens 42 causes beams 36a and 36b to intersect one another at ameasurement volume 44. Lens 42 also brings the beams to a focus at themeasurement volume. As the beams cross, they interfere with one anotherto form interference fringes, i.e. alternating regions of higher andlower intensity. Downstream of measurement region 44, beams 36a and 36bencounter a second focusing lens 46 which directs the beams to a beamstop 48.

The optical system further includes optical receivers for sensing laserenergy scattered by particles or other elements travelling through themeasurement volume. One of the optical receivers includes a convex lens50 for collecting and columinating scattered light, a photodetector 52,and a convex lens 54 between lens 50 and photodetector 52 for focusingthe collected light onto the photodetector. Photodetector 52, which canbe an avalanche photodiode, generates an analog voltage as a function ofreceived light. The other optical receiver is similar, and includes acollecting lens 56, a focusing lens 58 and a photodetector 60.

Both of the optical receivers are positioned off of the beam axis toinsure in each case that only scattered light reaches the collectinglens via a non-longitudinal path. As shown, collecting lenses 50 and 56are positioned to receive forward scattered light. The lenses can bepositioned to receive back scattered light, if desired. Each collectinglens is positioned so that its focal point occupies the measurementvolume. Further, lenses 50 and 56 are positioned such that therespective paths for scattered light to these lenses do not form thesame angle relative to the beam axis.

While two optical receivers are shown in FIG. 2, it is to be appreciatedthat a single optical receiver would suffice for measuring velocitiesbased on Doppler frequencies. Conversely, further optical receivers andtwo or more pairs of incident beams may be employed in situations ofdirectional ambiguity, or where measurements are taken in two or threedimensions. Examples of these situations include aerodynamic andturbulent flow studies and studies of atomized sprays.

The optical system detects light-scattering elements conveyed throughthe measurement region by a liquid or a gas. FIG. 2 shows a nozzle 62for admitting particle-containing air, into an enclosed chamber (notshown). The air is drawn out of the chamber through an exit conduit 66for a continuous flow downward as viewed in the figure. A pump (notshown) reduces pressure in conduit 66 to generate the flow, and iscontrolled to maintain a steady velocity of the air within the chamber.As seen in FIG. 3, the air flow causes a stream of particles 68 to flowthrough measurement volume 44.

While FIG. 2 illustrates a stream of solid particles carried by air, itis to be appreciated that the optical system can be used to sensedifferent types of discreet elements carried by different media, forexample liquid droplets within a gas, gas bubbles within a liquid, solidparticles within a liquid medium, or even light-scattering solidelements carried on a moving, light-absorptive solid medium. In anyevent, especially for particle size measurements the concentration ofdiscreet elements in the medium and the size of the measurement volumeinsure the presence of at most a single discreet element within themeasurement volume. For example, a fluid may include particles at aconcentration on the order of 10⁴ particles per cubic centimeter, withan appropriate measuring volume on the order of 100×1,000 microns.Frequently, sufficiently low concentrations occur naturally in fluidflows under test. If not, such flows are diluted before testing, e.g.with ultrapure water or gas. 0f course, the optical system elements canbe controlled to a certain extent to enlarge or reduce the size ofmeasurement volume 44 in accordance with expected concentrations.

The photodetector outputs are composite signals including backgroundnoise segments, and active segments corresponding to a light-scatteringelement's travel through the measurement volume. The active segmentscomprise the only useful or meaningful information of the photodetectoroutput signals. These active segments, called coherent frequency bursts,occur intermittently and at random. As noted above, the coherentfrequency bursts can vary considerably in their amplitude, frequency andduration. Accordingly, signal processing circuitry must distinguishcoherent frequency bursts from background noise, respond rapidly enoughto detect short bursts, and handle wide-ranging frequencies andamplitudes.

Filtering device 22 provides a broad frequency band for burst detection.As seen in FIG. 4, the filtering device includes sixteen bandpassfilters 70a-70p, encompassing different and overlapping frequencybandwidths. One of these filters is selected for bandpass filtering ofthe incoming signal. An operator can select a filter by providing afilter selection input as indicated at 72. Alternatively, filterselection logic 74 automatically selects one of filters 70 by comparingdata rates and selecting the filter generating the highest data rate. Ineither event, the selected bandpass filter eliminates high frequencynoise and the pedestal (d.c. component) of the burst. Table I lists theoverlapping frequency ranges for the bandpass filters in a preferredversion of filtering device 22:

                  TABLE I                                                         ______________________________________                                        Filter  Frequency Range                                                                             Filter   Frequency Range                                ______________________________________                                        70a      0.3 to 3 kHz 70i       3.0 to 20 MHz                                 70b      1.0 to 10 kHz                                                                              70j       5.0 to 35 MHz                                 70c      3.0 to 30 kHz                                                                              70k      10.0 to 50 MHz                                 70d     10.0 to 100 kHz                                                                             701      20.0 to 60 MHz                                 70e     30.0 to 300 kHz                                                                             70m      30.0 to 70 MHz                                 70f      0.1 to 1 MHz 70n      40.0 to 80 MHz                                 70g      0.3 to 3 MHz 70o      50.0 to 100 MHz                                70h      1.0 to 10 MHz                                                                              70p      10.0 to 100 MHz                                ______________________________________                                    

Thus, coherent frequency bursts within a broad range of about 0.3kHz-100 MHz are selectively bandpass filtered to enhance thesignal-to-noise ratio.

The output of filtering device 22 is provided to a resistive powersplitter 76. Amplifiers 78 and 80 provide the amplified output to burstdetector 24 and to frequency translator 26, respectively. In the mannerexplained in the aforementioned U.S. Pat. No. 4,973,969, burst detector24 distinguishes a coherent frequency burst from the background noise ofthe signal. Upon detecting a coherent frequency signal, burst detector24 generates two output signals: burst gate signal 27, and frequencyrange signal 29. The burst gate signal is a one-bit signal active("high" logic level) when burst detector 24 is receiving a coherentfrequency signal from amplifier 78. Frequency range signal 29 is afour-bit digital word indicating an approximate frequency of the burst.The estimated frequency is obtained with multiple, incremented delaylines, in the manner described in the '969 patent.

With reference to the bandwidth over which burst detector 24 receivessignals, the four bit frequency range signal is capable of dividing thatbandwidth into sixteen different bandwidth segments. However,satisfactory results have been achieved by dividing the burst detectorbandwidth into four such bandwidth segments. For example, an incomingdecade bandwidth is divided into four one-octave bandwidth segments.Table II illustrates the segmenting of the bandwidth of filter 70h, i.e.1-10 MHz.

                  TABLE II                                                        ______________________________________                                        Selected filter: 1-10 MHz                                                     Base sample frequency: F = 50 MHz                                             Range    0000-0011 0100-0111 1000-1011                                                                             1100-1111                                (binary):                                                                     Bandwidth                                                                              .625-1.25 1.25-2.5  2.5-5   5-10                                     Segment                                                                       (MHz):                                                                        Divide F:                                                                              F/8       F/4       F/2     F                                        Sampling 6.25 MHz  12.5 MHz  25 MHz  50 MHz                                   Frequency:                                                                    ______________________________________                                    

In the same manner, each of the other bandpass filters 70 has anassociated base sampling frequency F, which is either applied directlyor divided in accordance with the appropriate one of four bandwidthsegments. Table III illustrates base frequencies and bandwidth segmentsfor bandpass filters 70b through 70i, with the final four columnsindicating the frequency range signal (numerical rather than binaryrange) and the factor by which the base frequency is divided.

                  TABLE III                                                       ______________________________________                                              Band            0-3:         8-11:                                      Filter                                                                              width   Base F  F/8   4-7:F/4                                                                              F/2    12-15:F                             ______________________________________                                        70b   1-10    50 kHz  .625- 1.25-  2.5-5  5-10                                      kHz             1.25  2.5 kHz                                                                              kHz    kHz                                                       kHz                                                     70c   3-30    .15     1.87- 3.75-  7.5-15 15-30                                     kHz     MHz     3.75  7.5 kHz                                                                              kHz    kHz                                                       kHz                                                     70d   10-100  .5      6.25- 12.5-25                                                                              25-50  50-100                                    kHz     MHz     12.5  kHz    kHz    kHz                                                       kHz                                                     70e   30-300  1.5     19-37.5                                                                             37.5-75                                                                              75-150 150-300                                   kHz     MHz     kHz   kHz    kHz    kHz                                 70f   .1-1    5 MHz   62-125                                                                              125-250                                                                              250-500                                                                              .5-1                                      MHz             kHz   kHz    kHz    MHz                                 70g   .3-3    15      187-  370-750                                                                              .75-1.5                                                                              1.5-3                                     MHz     MHz     375   kHz    MHz    MHz                                                       kHz                                                     70h   1-10    50      .62-  1.25-  2.5-5  5-10                                      MHz     MHz     1.25  2.5 MHz                                                                              MHz    MHz                                                       MHz                                                     70i   3-20    100     N/A   2.5-5  5-10   10-20                                     MHz     MHz           MHz    MHz    MHz                                 ______________________________________                                    

Thus, for the selected bandpass filter 70, frequency range signal 29indicates one of four bandwidth segments and causes selection of theappropriate sampling frequency rate. Upon comparing each base samplingfrequency F (or the result of the indicated division of F) with itsassociated bandwidth segment, it will be appreciated that so long as theactual frequency lies within the indicated bandwidth segment, theincoming signal will be sampled at a rate of 5-10 samples per cycle.

FIG. 5 shows frequency translator 26 in greater detail. Translator 26includes an analog-to-digital converter 82 that receives an analogvoltage signal V_(i) from amplifier 80, and converts V_(i) to a binaryword or digital value representing the voltage. A/D converter 82 is aneight-bit device that generates an eight-bit binary word upon eachsampling of V_(i). The A/D converter output is a sequence of the binarywords, each representing the voltage at a different sampling time. Thebinary words can represent voltage values over 256 increments, with 255(binary 11111111) representing the highest expected voltage and binaryzero representing the most negative voltage, with zero voltagecorresponding to 128.

The output of A/D converter 82 is stored to a sequentialfirst-in-first-out memory 84. The memory has an eight-bit width (eightchannels) corresponding to the length of the binary words and has alength (in number of bit positions) sufficient to encompass the maximumexpected burst length. In the preferred embodiment, the length of memory84 is 1024.

The contents of memory 84 are read out to a digital-to-analog converter86 having eight input lines for converting the sequence of digitalvalues into an analog signal. The signal from D/A converter 86 isprovided to a low pass filter 88 with a cutoff frequency set at theNyquist value, i.e. at a maximum of one-half of the frequency at whichD/A converter 86 converts the digital value sequence. The output V_(o)of low pass filter 88 is a reconstructed, frequency-translated analogsignal representing the burst.

Frequency translator 26 further includes a controller 90. Inputs to thecontroller include burst gate signal 27 and frequency range signal 29from the burst detector; a bandwidth filter selection input frommicroprocessor 30; and two clocking inputs of 30 MHz and 100 MHz,respectively indicated at 92 and 94. Based on these inputs, controller90 provides a sampling clock input to A/D converter 82 and to memory 84via a line 96. The controller provides a read frequency via a line 98 asa clocking input to the memory and to D/A converter 86. Furthercontroller outputs include a reset signal 100 to memory 84 and abandpass filter select signal 72 to filtering device 22. Controller 90preferably is a programmable sequencer or a programmable logic device.

A salient feature of the present invention is that despite an initialburst capture range of several orders of magnitude in frequency, and abandpass filter output V_(i) that can range over a decade in frequency,the translated analog signal V_(o) is confined to a frequency range ofone octave. This result is achieved through selection and control of theclock rate at which V_(i) is sampled by A/D converter 82 and stored tomemory 84, in conjunction with the clock rate at which the series ofdigital values is read out of the memory and converted by D/A converter86.

Semiconductor logic in controller 90, actuated when the burst gatesignal goes high, selects one of four available sampling clock rates asa function of: (1) the four-bit binary frequency range signal; and (2)the selected bandwidth filter, whether such selection was automatic orby the operator. The available sampling clock rates are generated byclocks 92 and 94, and depend upon the bandwidth filter selected. Forexample, the bandwidth of 30-300 kHz has sampling rates that correspondto the following adjacent frequency segments or ranges: 19-37.5 kHz;37.5-75 kHz; 75-150 kHz and 150-300 kHz. Likewise, each of the remainingburst detector bandwidths corresponding to remaining bandwidth filters70, is divided into four bandwidth segments. Each preferably encompassesone octave.

Although coherent frequency bursts can differ considerably with oneanother as to their frequencies, each burst considered alone issubstantially uniform in its frequency. Consequently, a sufficientlyaccurate estimate of burst frequency is obtained within a few cycles ofinitial detection.

The controller logic selects the sampling frequency based on apredetermined desired number of samples per cycle of the incoming analogsignal. For example, if filter 70p was the selected filter (10-100 MHz),and the frequency range signal indicated a burst frequency in the rangeof 10-20 MHz, then selecting a sampling frequency of 100 MHz wouldpredetermine a number of samples per cycle ranging from 5 to 10,depending upon the actual burst frequency. The number will lie withinthe range of 5-10, so long as the actual burst frequency is within therange of 10-20 MHz. The range of 5-10 is preferred, although as few as2.5 samples per cycle can be satisfactory in certain velocitymeasurement applications.

The sampling frequency enables A/D converter 82, controls the rate atwhich the A/D converter samples the incoming analog signal V_(i), andalso sets the same rate for storing the digital values to memory 84. Toinsure that memory 84 is cleared for accepting the digital values,controller 82 provides reset pulse 100 to the memory, responsive to theburst gate signal going high. So long as the gate signal remains high,signal sampling and storage proceed at the selected sampling frequency.

The signal sampling mode is illustrated in FIGS. 6 and 7, respectivelyfor a high frequency analog burst signal 102a and a low frequency analogburst signal 102b. Respective burst gate signals 104a and 104b go highwithin the first several cycles of the detected burst. Respectivesampling frequencies are shown at 106a and 106b. In each case, thesampling frequency is selected to yield a desired number or limitedrange of samples per cycle of the burst. Since the frequency of analogburst 102a is approximately four times the frequency of analog burst102b, clocking signal 104a is likewise about four times the clockingfrequency 104b.

Returning to FIG. 5, burst gate signals 104 go inactive or "low" inresponse to detecting the end of the coherent frequency burst. Inresponse, controller 90 shifts from the burst signal sampling mode to aread mode in which the stored binary words are read out of memory 84 andprovided to D/A converter 86, in the same sequence in which they werestored. Reading occurs at a read frequency predetermined by thecontroller. Accordingly, respective digital read signals 108a and 108bhave the same frequency, preferably about 40 MHz. Respectivefrequency-translated analog signals V_(o) are shown at 110a and 110b inFIGS. 6 and 7.

Analog signals 110a and 110b do not necessarily have the same frequency.However, the higher frequency signal will have a frequency at mostdouble the lower frequency, regardless of the ratio of frequencies forincoming analog signals 102a and 102b.

The selected bandpass filter 70 and the selected sampling frequency 106are stored in microprocessor 30. Accordingly, despite the similarity inthe frequencies of reconstructed signals 110a and 110b, microprocessor30 calculates and causes terminal 32 to display the true frequencies ofthe original bursts.

FIG. 8 discloses an alternative embodiment frequency translator in whichtwo A/D converters, phase-adjusted relative to one another, are employedto increase the frequency at which incoming signal V_(i) is sampled.More particularly, signal V_(i) is provided to an A/D converter 101 andalso to an A/D converter 103. A delay line 105 is configured tophase-shift a sample clock, so that the sampling of the signal providedto A/D converter 103 lags the sampling of the signal provided to A/Dconverter 101 by 180 degrees. A/D converter 101 provides its digitaloutput to a sequential memory 107, while A/D converter 103 similarlyprovides its output to a sequential memory 109. A controller 111receives two clocking inputs of 15 MHz and 50 MHz from clock oscillatorsindicated at 113 and 115, respectively. The output of delay line 105 isprovided to sequential memory 109 as well as A/D converter 103.Accordingly, memory 109 receives the A/D converter output at the 180degree phase shift with respect to storage of the A/D converter outputto sequential memory 107.

Considering just oscillator 115 (50 MHz), the maximum frequency of theresultant controller clocking signal to the A/D converters and thesequential memories is 50 MHz. However, because of the half-cycle phaseshift of the clocking signal provided to converter 103 and memory 109relative to the clocking signal provided to converter 101 and memory107, the effective rate at which V_(i) is sampled, is 100 MHz. In thesame manner, the frequency of oscillator 113 is effectively doubled to asampling rate of 30 MHz.

Controller 111 alternately reads sequential memories 107 and 109 to aD/A converter 117. The D/A converter provides its analog output to alow-pass filter 119, the output of which is the frequency-translatedburst signal V_(o). At any given time, only one of the sequential memoryoutputs is enabled to provide input to D/A 117. In other words, theoutputs of sequential memories 107 and 109 are interleaved when providedto the D/A converter. Each sequential memory is read at a frequency of20 MHz, resulting in a D/A converter input frequency of 40 MHz.

At present, this alternative translator is preferred, in spite of therequirement of an additional A/D converter and sequential memory.Oscillators 113 and 115 are less expensive than their counterpartoscillators 92 and 94. Further, the phase-shifted, lower frequencysignals are more robust for enhanced accuracy and reliability.

FIG. 9 shows signal processing circuitry of a phase Doppler deviceconstructed according to the present invention. Device 112 is useful indetermining the sizes of particulates, aerosols and droplets in sprays.With reference to FIG. 2, device 112 translates frequencies of analogfrequency bursts generated by photodetectors 52 and 60, then measuresthe phase difference between the respective frequency-translated signalsas an indication of particle or droplet size.

For providing the necessary inputs to its frequency translatingcircuitry, device 112 employs a bandpass filtering device and burstdetector with the photodetectors, each similar to its counterpart shownin FIG. 1. The amplified and bandwidth filtered analog signals areprovided, respectively, to A/D converters 114 and 116. A/D converter 114samples the input and stores the digital values to a memory 118. In theread mode, the contents of the memory are provided to a D/A converter120. The D/A converter output is low-pass filtered at 122 and providedto a comparator amplifier 124.

Likewise, A/D converter 116 stores its output to a memory 126, whosecontents are later read into a D/A converter 128, whereupon the D/Aconverter output is low-pass filtered at 130 and provided to acomparator amplifier 132. A controller 134 provides a sampling frequencyin common to both A/D converters and both memories, and likewiseprovides a common read frequency to the memories and the D/A converters.The controller responds to the same input signals (not shown in FIG. 9)as are provided to controller 90 in FIG. 5.

Each of amplifiers 124 and 132 has a threshold voltage input V_(t). Theamplifier outputs are provided through respective capacitors 136 and 138to a phase detector 140 for phase detection. The analog phase detectoroutput is provided to an A/D converter 142, which also includes anenabling input 144 from controller 134. The A/D converter output isprovided to a microprocessor 146 via an interface 150. A displayterminal 148 connected to the microprocessor displays information on thesize of particles, droplets or aerosols, based on the phase differenceinput. Microprocessor 146 is coupled to controller 134 through theinterface.

FIG. 10 shows signal processing circuitry of a velocity measurementsystem 152 which includes optical components such as those shown in FIG.2, but requires only one of photodetectors 52 and 60. The system alsoutilizes a burst detector and filtering device as previously discussed.The bandwidth filtered and amplified input V_(i) is sampled by an A/Dconverter 154, and the resulting digital values are stored in sequenceto a memory 156. A controller 158 governs the sampling and storing ratebased on the selected bandwidth filter and the estimated burst frequencyrange. The controller further governs the rate at which the sequence ofbinary words is read out of the memory to a D/A converter 160. The D/Aconverter output is low-pass filtered at 162, and provided to acomparator amplifier 164. A threshold input V_(t) is also provided tothe amplifier.

The amplifier output is provided to a shift register 166, governed by aclock rate from the controller via a line 168. The shift register outputis provided to an autocorrelation circuit 170, where a digitalcorrelation of the burst signal is constructed. The output of theautocorrelator is provided to a microprocessor 172 via an interface 174.

While the disclosed embodiments are preferred, it is to be recognizedthat alternative components can be selected to provide several of thedisclosed functions. For example, those skilled in the art are aware ofmeans other than the burst detector, for estimating the burstfrequencies and frequency ranges. One-bit digitizing could be employedin lieu of the multibit digitizing of the A/D converters. However, thissubstitution would result in reduced phase resolution and would requirea greater number of samples per burst cycle, e.g. at least ten samplesper cycle. An addressable memory register could be employed in lieu ofthe sequential FIFO memory. Also, digital values read out of the memorycould be subject to direct processing, e.g. a fast Fourier transform.However, translation back to an analog signal is preferred at present,since less time is required for signal processing.

Thus in accordance with the present invention, analog coherent frequencysignals received over a broad range of frequencies are converted todigital information, then reconstructed as analog signals confined to amuch narrower frequency bandwidth. The reconstructed signals can beprocessed with simpler, less costly circuit components, yet provide moreaccurate and reliable measurements of physical characteristics, becausesignal processing is confined to the narrowed frequency bandwidth. Thus,analog and digital processing techniques are combined in a unique mannerthat affords rapid frequency bandwidth compression and better preservesthe signal-to-noise ratio.

What is claimed is:
 1. An apparatus for detecting and measuring coherentfrequency bursts of a composite signal including the coherent frequencybursts and background noise, said apparatus including:a detection meansreceiving the composite signal and having a detector bandwidth toaccommodate coherent frequency bursts of the composite signal, forgenerating an enabling signal whenever receiving a coherent frequencyburst, and for generating a frequency range value indicating anestimated burst frequency of the coherent frequency burst in cycles perunit time; an analog-to-digital conversion means, receiving thecomposite signal, for sampling the coherent frequency burst to convertthe coherent frequency burst to a digital burst signal; a memoryoperatively associated with the analog-to-digital conversion means, saidanalog-to-digital conversion means storing the digital burst signal tothe memory as a sequence of digital values, each digital valuecorresponding to one of the samples of the coherent frequency burst; aread means operatively associated with the memory, for reading thedigital burst signal out of the memory in said sequence, to generate areconstructed digital burst signal; and a control means operativelyassociated with the detecting means, the analog-to-digital conversionmeans, the memory and the read means, for selecting one of a pluralityof different sampling rates in samples per unit time, each sampling ratebeing selected as a function of the frequency range value of itsassociated one of the frequency bursts and further being selected uponreceiving the frequency range value and the enabling signal, whereineach one of the sampling rates is associated with a different bandwidthsegment of the detector bandwidth, and wherein the respective samplingrates are generally proportional to the frequencies within theirassociated bandwidth segments; said control means applying the selectedsampling rate to the analog-to-digital conversion means and to thememory, to cause the analog-to-digital conversion means to sample theassociated coherent frequency burst at said selected sampling rate andto store the digital burst signal to the memory at the selected samplingrate; said control means further applying a predetermined read rate insamples per unit time to the read means, to cause the read means to readthe digital burst signal sequentially from the memory at said readfrequency, said read rate being substantially constant so that thereconstructed digital burst signals have respective resultantfrequencies, in cycles per unit time, as a function of the read rate andthe respective selected sampling rates, whereby said resultantfrequencies are confined to a resultant frequency range substantiallynarrower than said detector bandwidth.
 2. The apparatus of claim 1including:a frequency band pass filtering means receiving the compositesignal and providing a filtered composite signal to the detecting meansand to the analog-to-digital conversion means.
 3. The apparatus of claim1 wherein:the analog-to-digital conversion means comprises a multibitanalog-to-digital converter that generates each said digital value as amultiple bit binary word upon each sampling of the coherent frequencyburst.
 4. The apparatus of claim 3 wherein:the memory comprises asynchronous first-in-first-out memory having a plurality of bit channelsequal in number to the bit length of the binary words.
 5. The apparatusof claim 1 wherein:the analog-to-digital conversion means comprises aplurality of multibit analog-to-digital converters, each of saidanalog-to-digital converters generating a multiple bit binary word eachtime it samples the coherent frequency burst, said analog-to-digitalconverters being driven with respective clocking signals having the sameclock rate but phase-shifted relative to one another, whereby saidselected sampling rate is equal to the clock rate multiplied by thenumber of analog-to-digital converters.
 6. The apparatus of claim 5wherein:the memory comprises a plurality of synchronousfirst-in-first-out memories, each of the memories receiving the outputof an associated one of the analog-to-digital converters.
 7. Theapparatus of claim 1 wherein:a frequency ratio of the highest frequencyof the detector bandwidth to the lowest frequency of the detectorbandwidth, is at least three times a frequency ratio of the highestfrequency to the lowest frequency in each of the bandwidth segments. 8.The apparatus of claim 7 wherein:said frequency ratio of the detectorbandwidth is about ten and said frequency ratio in each of the bandwidthsegments is about two.
 9. The apparatus of claim 7 wherein:each saidsampling rate is predetermined with respect to its associated bandwidthsegment to insure a sampling rate in the range of 5-10 samples in eachcycle of the coherent frequency burst.
 10. The apparatus of claim 1further including:a digital-to-analog conversion means operativelyassociated with the memory to receive the reconstructed digital signal,for converting the reconstructed digital signal to a reconstructedanalog burst signal.
 11. The apparatus of claim 10 further including:alow pass filtering means operatively associated with thedigital-to-analog conversion means, for low pass filtering thereconstructed analog burst signal.
 12. The apparatus of claim 1 furtherincluding:a storage means for retaining a record of the selectedsampling rate, and logic means for determining the burst frequency basedupon the sampling rate and the frequency of the reconstructed digitalburst signal.
 13. The apparatus of claim 1 wherein:the control meanscomprises a programmable sequencer.
 14. A process for measuring acoherent frequency analog signal, including the steps of:receiving ananalog coherent frequency signal, and generating a frequency range valueindicating an approximate frequency in cycles per unit time of thereceived analog coherent frequency signal; predetermining a plurality ofsignal sampling rates in samples per unit time, each sampling ratecorresponding to a different one of a plurality of frequency bandwidthsegments that together provide a frequency bandwidth for receiving theanalog coherent frequency signal, said sampling rates being generallyproportional to the frequencies within their associated bandwidthsegments; selecting one of the plurality of signal sampling rates, basedon the frequency range value; converting the analog coherent frequencysignal to a first digital signal by sampling the analog coherentfrequency signal at the selected sampling rate; storing the firstdigital signal to a memory as a sequence of digital values, each digitalvalue corresponding to one sample of the analog coherent frequencysignal; reading the first digital signal out of the memory in saidsequence and at a predetermined read rate, to generate a second digitalsignal having a resultant frequency based on the selected sampling rateand the read rate, said read rate being substantially constant toconfine the resultant frequency to a frequency range substantiallynarrower than said frequency bandwidth; and converting the seconddigital signal to an analog signal, to generate a reconstructed analogcoherent frequency signal having said resultant frequency.
 15. Theprocess of claim 14 wherein:the step of converting the analog coherentfrequency signal to the first digital signal comprises generatingmultiple eight-bit binary words, each of the first digital valuesconsisting of one of the eight-bit binary words.
 16. The process ofclaim 14 including the further step of:predetermining the range of eachsaid bandwidth segment, such that the ratio of the highest frequency tothe lowest frequency in the bandwidth segment is at most two.
 17. Theprocess of claim 16 including the further step of:selecting each of thesampling rates with reference to the bandwidth segment represented byits associated frequency range value, such that the resulting samplingrate has a frequency of at least 2.5 times the frequency of the analogcoherent frequency signal.
 18. The process of claim 14 including thefurther step of:low pass filtering the reconstructed analog coherentfrequency signal.
 19. The process of claim 14 further including the stepof:measuring said resultant frequency and determining the frequency ofthe received analog coherent frequency signal based on the resultantfrequency and the selected sampling rate.
 20. The process of claim 14wherein:the coherent frequency analog signal is part of a compositesignal that further includes background noise; said steps of generatinga frequency range value, selecting one of the signal sampling rates,converting the analog coherent frequency signal, and storing the firstdigital signal, all are performed only while a detection means,responsive to sensing the analog coherent frequency burst within thecomposite signal, generates an enabling signal; and the step of readingthe first digital signal out of the memory is not initiated until thedetecting means ceases to generate the enabling signal.
 21. A particlemeasuring device, including:a means for generating two linearlypropagated coherent energy beams, and a beam guide means for causing thebeams to intersect one another at a predetermined beam angle andinterfere with one another over a measuring region at theirintersection; a particle moving medium for carrying particles throughthe measuring region, each of the particles scattering the coherentenergy as the particle passes through the measuring region; a firstenergy detecting means for receiving a portion of the scattered coherentenergy and for generating a first analog signal comprising a coherentfrequency burst as a function of the received coherent energy; a firstdetection means for receiving the first analog signal and for generatinga first frequency range value to indicate an approximate burst frequencyof the first analog signal; a first analog-to-digital conversion meansreceiving the first analog signal, and for converting the first analogsignal to a first digital signal; a first memory operatively coupled tothe analog-to-digital conversion means, for storing the first digitalsignal; a first read means operatively associated with the memory forreading the first digital signal sequentially out of the memory at asubstantially constant read rate; a control means operatively associatedwith the first detection means, the first analog-to-digital conversionmeans, the first memory, and the first read means, for selectivelygenerating one of a plurality of sampling frequencies as a function ofthe first frequency range value, each sampling frequency correspondingto a different one of several first frequency bandwidth segmentscooperating to provide a frequency bandwidth of the first detectingmeans, wherein the first frequency bandwidth encompasses a substantiallywider range of frequencies than does any one of the first frequencybandwidth segments; the control means applying the selected samplingrate to the first analog-to-digital conversion means, to cause the firstanalog-to-digital conversion means to sample the first analog signal atthe selected sampling rate when so converting, and to store the firstdigital signal to the memory as a sequence of first digital values, eachfirst digital value corresponding to one of the samples; the controlmeans further causing the first read means to read the stored data fromthe first memory in said sequence and at the read rate to generate asecond digital signal at a resultant frequency based on the read rateand the selected sampling rate, whereby the resultant frequency lieswithin a frequency range substantially narrower than the first frequencybandwidth; and a digital-to-analog conversion means for converting thesecond digital signal to a second analog signal comprising areconstructed coherent frequency burst having said resultant frequency.22. The apparatus of claim 21, further including:a means for determiningthe frequency of the first analog signal, based on the resultantfrequency and the selected sampling rate; and a means for determiningthe velocity of the particle, based on the frequency of the first analogsignal.
 23. The apparatus of claim 21 wherein:the first detecting meansincludes a bandpass filtering means comprised of a plurality ofbandwidth filters, and means for individually selecting one of thebandwidth filters to set the detecting means bandwidth.
 24. Theapparatus of claim 21 wherein:the ratio of the highest frequency to thelowest frequency within each of the bandwidth segments is at most abouttwo.
 25. The apparatus of claim 24 wherein:the sampling rates arepredetermined with respect to their corresponding bandwidth segments,such that each sampling rate is at most ten times and at least 2.5 timesthe frequency of the first analog signal.
 26. The apparatus of claim 21further including:a means for low pass filtering the second analogsignal.
 27. The apparatus of claim 21 further including:a secondphotodetecting means spaced apart angularly from the firstphotodetecting means, for generating a third analog signal comprising acoherent frequency burst signal based on the scattered coherent energy;a second analog-to-digital conversion means receiving the third analogsignal, for converting the third analog burst signal to a third digitalsignal; and a second memory operatively associated with the secondanalog-to-digital conversion means, for storing the third digitalsignal; a second read means operatively associated with the secondmemory, for reading the third digital signal sequentially out of thesecond memory at the read rate; said control means further beingoperatively associated with the second detecting means, the secondanalog-to-digital conversion means, the second memory and the secondread means, for causing the second analog-to-digital conversion means tosample the second analog signal at the selected sampling rate when soconverting, and causing the second read means to read the third digitalsignal sequentially out of the second memory at the read rate to providea fourth digital signal; a second digital-to-analog conversion meansoperatively associated with the second read means and the second memory,for converting the fourth digital signal into a fourth analog signal;and a phase detecting means for determining a phase difference betweenthe second analog signal and the fourth analog signal, and means fordetermining the size of the particle based on the phase difference. 28.The apparatus of claim 27 further including:a means for low-passfiltering the second analog signal.
 29. A process for detecting andmeasuring coherent frequency signals, including:using a first detectorto detect analog coherent frequency signals having frequencies within afirst frequency range, and for each of the analog coherent frequencysignals so detected:(a) generating a frequency value representing anapproximate frequency of the analog coherent frequency signal; (b)sampling the analog coherent frequency signal at a sampling rate togenerate a sequence of digital values, and storing the digital values insaid sequence to a digital memory; (c) reading the digital values out ofthe digital memory in said sequence and at a read rate, to provide areconstructed digital signal; (d) converting the reconstructed digitalsignal to a reconstructed analog signal; and (e) selecting a ratio ofthe sampling rate to the read rate corresponding to the frequency value;and controlling the sampling rate and the read rate whereby saidselected ratios increase as the frequency values increase and decreaseas the frequency values decrease, to confine the reconstructed analogsignals within a second frequency range substantially narrower than thefirst frequency range.
 30. The process of claim 29 furtherincluding:using a second detector to detect the analog coherentfrequency signals; performing steps (a)-(e) with respect to said seconddetector, to provide further reconstructed analog signals; anddetermining a difference in phase between the reconstructed analogsignal and the further reconstructed analog signal corresponding to eachanalog coherent frequency signal.
 31. The process of claim 29wherein:said step of controlling the sampling rate and the read rateincludes varying the sampling rate based on changes in the frequencyvalues while maintaining said read rate constant.
 32. The process ofclaim 31 wherein:said step of varying the sampling rate includessegmenting said first frequency range into a plurality of frequencyrange segments, each said segment corresponding to a different frequencyvalue, providing a plurality of predetermined sampling rates with eachof the sampling rates corresponding to one of the frequency rangesegments, and selecting the sampling rate corresponding to the frequencyrange segment in which the analog coherent frequency signal is detected.33. The process of claim 29 wherein:the size of said first frequencyrange is at least three times the size of the second frequency range,with the size of each range being a ratio of the highest frequency tothe lowest frequency within that range.